Apparatus for locking a door in a closed position are numerous and varied in design and complexity. The shortcoming of most locking apparatus is that they only operate to secure the door when the door is in the closed position. If the door should be opened to view objects on the other side or to pass small items through the doorway, most locking apparatus will not prevent the door from being forced completely open by an unwanted intruder.
Door chains connected to the door frame and selectively connected to the door provide a means for limiting the movement of a door from a closed position. However, most door chains are limited in the amount of force exerted on a door that the chain can withstand. The chain must be connected to the door itself, which provides a minimal structural base for securing the chain and further, the force exerted on the door by an intruder is concentrated at the connection of the chain with the door. Most chains are detachably engaged by brackets connected to the door by screws or other fasteners common to the industry. The force of an intruder is concentrated on the fasteners which usually have a minimal cross-sectional area and/or a minimal securing in contact with the door. Screws, bolts, nails and other securing apparatus tend to strip from the door when a large force ( i.e. the mass of a human body in motion) is exerted against the door. Even if the securing apparatus were to hold, it is likely that the concentration of force on such a small area of the door will break the portion of the door to which the chain is attached, thus permitting the door to open.
Another method of limiting the movement of the door from a closed position is the use of door braces which are pivotally secured at a lower end to the floor and extend in angular relation to and in abutment with the door. An upper end of the door brace is received within a vertically extending slot in the door such that movement of the door from the closed position will urge the upper end of the brace to the top of the slot whereby the brace will contact a stop and thus resist further movement of the door. The problem with door braces is twofold. One, the force exerted by an intruder is still concentrated at a very small area of the door and if the door is wooden, as many doors are, the door brace could be driven through the door. Secondly, the door brace assembly extends some distance from the door and within the adjacent room, thus presenting a visually distracting sight and possibly a hazardous obstacle to an inattentive person who may inadvertently trip over the brace.
A recent improvement in doorstops, commercially know as the "Door Club", is a floor mounted device which allows an occupant to prevent or limit the ability of a potential intruder to gain a forced entry through the door. This device comprises a shaft adapted to be slidably received in a tubular receptacle seated within a hole in the floor near the door. The shaft extends upwardly into the pathway of the door, effectively limiting movement of the door. Integral with the shaft are parallel horizontal cross members joined to one another at their ends by two semi-circular portions which are parallel to one another but which project horizontally at right angles to the parallel cross members. When the shaft is slid into the receptacle with the semi-circular portions extending towards the door, the semi-circular portions contact the closed door and prevent opening of the door. If the shaft of the "Door Club" is removed from the receptacle, is rotated 180 degrees about its axis and is reinserted into the receptacle, the semi-circular portions then face away from the door whereupon the door can be opened a small but finite distance until it contacts the shaft and/or the horizontal cross-members integral therewith, whereupon further movement of the door is arrested. During the brief interval when the "Door Club" has been removed from the receptacle to reverse it, the door is vulnerable to a sudden force which could open it.
A modification of the doorstop is adapted for use on sliding doors of the type typically used to join the interior of a building with an outside deck or patio. In this application, a plurality of holes are located along the door track, with the hole closest to the door when in a closed position adapted to coact with the doorstop to prevent any movement of the door when closed. The door may be opened progressively wider by removing the doorstop from the first hole and placing it in one of the other holes. Because many sliding doors are made of glass, the observer on the other side of the door can readily observe when the doorstop has been removed from one hole and can suddenly force the door open before the building occupant has an opportunity to place the doorstop in the second hole.